A shaped charge suitable for use in a perforating tool for a subterranean well is described. The invention relates particularly to an improved shaped charge liner constructed from compressed powdered heavy metal and polymer material.
A subterranean gas or oil well typically begins with a hole bored into the earth, which is then lined with joined lengths of relatively large diameter metal pipe. The casing thus formed is generally cemented to the face of the hole to give the well integrity and a path for producing fluids to the surface. Conventionally, the casing and cement are subsequently perforated with chemical means, commonly explosives, in one or more locations of the surrounding formation from which it is desired to extract fluids. In general, the perforations extend a distance into the formation. One of the problems inherent in the art is to maximize the depth of penetration into the formation.
Explosive shaped charges known in the art generally have a substantially cylindrical or conical shape and are used in various arrangements in perforating tools in subterranean wells. Generally, a tubular perforating gun adapted for insertion into a well is used to carry a plurality of shaped charges to a subsurface location where perforation is desired. Upon detonation of the shaped charges, explosive jets emanate from the shaped charges with considerable velocity and perforate the well casing and surrounding formation.
Liners of shaped charges have commonly been designed in an effort to maximize penetration depth. Various metals have been used. Solid metal liners have the disadvantage of introducing metal fragments into the formation, detracting from the effectiveness of the perforation. In order to overcome this problem, compressed powdered metal liners have sometimes been used. Such liners disintegrate upon detonation of the shaped charge, avoiding the problems associated with metal fragments. It is known in the art that heavy metals are particularly suited for use in liners. Generally, the heavy metal is combined with one or more other metals with suitable binding characteristics to improve the formation of rigid liners through very high compression of the metal powders. One of the principal problems in the art has been the attempt to increase the heavy metal content of liners. Such attempts are outlined in U.S. Pat. Nos. 5,656,791 and 5,814,758, which are incorporated herein for all purposes by this reference.
Success in the art of producing compressed powdered heavy metal liners has been limited by efforts to identify suitable binding agents among elemental metals and alloys. A particularly serious problem is encountered since the material properties of the various constituents of the metal powder can vary, specifically, particle size, particle shape, and particle density. The blending of the mixture must be done very carefully to avoid segregation of the powder constituents resulting in a poorly performing liner. Further difficulties are encountered with powdered metal liners in that the metals are subject to corrosion. Efforts have been made to coat the completed liners with oil or other material to inhibit corrosion. These efforts have met with imperfect success. Another problem with powdered metal liner known in the art has been the need for added lubricant to facilitate manufacturing the pressed liners. Commonly, powdered graphite is added to the powdered metal mixture, which necessarily reduces the quantity of heavy metal that can be included in the finished liner.
After much research and study, the present invention employs various polymers in combination with heavy metal powders to produce an improved shaped charge compressed liner. The invention facilitates a higher heavy metal content resulting in improved liner performance. The liners of the invention also have improved corrosion resistance and a decreased need for lubricant additives.
The inventions provide shaped charge apparatus for use in a subterranean well. In general, the inventions contemplate an improved liner for a shaped charge constructed from a combination of powdered metal and selected polymer material.
According to one aspect of the invention, a mixture of powdered heavy metal and powdered polymer binder is compressively formed into a rigid shaped charge liner.
According to another aspect of the invention, a liner for a shaped charge is constructed of a polymer-coated heavy metal powder compressively formed into a rigid shaped charge liner.
According to still another aspect of the invention, a liner for a shaped charge is constructed from a mixture of powdered heavy metal and powdered polymer binder blended with a polymer-coated heavy metal powder and compressively formed into a substantially conical rigid body.